Relatively small rubber products, such as rubber vibration insulators or the like for automobiles, are manufactured by injection molding in general. In most cases, a preplasticizing type of injection molding machine or inline screw injection molding machine is used.
A preplasticizing type of injection molding machine A, as shown in FIG. 14, comprises a plasticizing mechanism 70 for plasticizing a molding material, or rubber (unvulcanized rubber) and feeding it to the front portion of an injection cylinder 61, and an injection device 60 for injecting rubber being fed into a mold 40 and pressing it, such machines being classified into the plunger preplasticizing type and the screw preplasticizing type according to how rubber is pressed in.
An injection molding machine B of the inline screw type, as shown in FIG. 15, which is also referred to as the reciprocating screw type, has an injection device 60 provided with a screw 62 for plasticizing and measuring a molding material, or rubber, said screw 62 also performing the function of a plunger to inject the rubber resided in the front portion of an injection cylinder 61 into a mold 40 to fill the latter therewith.
Each of these injection molding machines is provided with a rubber injection molding nozzle 51 at the front end of the injection cylinder 61 of the injection device 60.
This nozzle 51, as shown in FIG. 16, has an injection channel 55 extending, through the nozzle 51, from the side of an attaching portion 52 at which the nozzle is attached to said injection cylinder 61, to the front end 51a. The nozzle 51 also has an orifice 56 which is disposed somewhere between the inlet 53 at the attaching side 52 of said channel 55 and the outlet 54 at the front end or which is disposed at the front end. This orifice 56 serves to increase the flow velocity of the rubber by reducing the channel diameter. Thereby, the heated rubber extruded from the cylinder 61 flows through said channel 55, having its flow velocity increased by the effect of constricting by said orifice 56, so that it is injected at high speed from the outlet 54 at the front end into the sprue 41 of the mold 40.
In this connection, in the industry using such rubber products produced by injection molding, particularly the automobile parts industry, cost reduction is urgent requirement and measures to meet this requirement are being investigated. As a means therefor, it could be contemplated to enhance internal heat generation of the rubber passing through the nozzle so as to increase the temperature of the rubber being injected, thus shortening the vulcanization time in the mold.
However, in the conventional nozzle 51 shown in FIG. 13 used in injection molding of rubber vibration insulator for automobile, the cross sectional shape of said orifice 56 is a simple circle, so that although more or less heat generation is brought about by passage of the rubber through said orifice 56, the effect of the vulcanization time reduction due to such heat generation is small.
In particular, the peripheral portion of the rubber passing through the orifice 56 receives a force of friction against the interior wall face of the orifice and the shearing stress in the rubber thereby increases; thus, the temperature of generated heat, though increased to some extent, is not very high in the innermost portion, which means that there are differences in temperature within the rubber being injected. Therefore, in order to ensure thorough vulcanization within the rubber, it is necessary to set the vulcanization time on the basis of this low temperature rubber portion, presenting a problem that the vulcanization time is correspondingly prolonged.